Cr-bearing heat-resistant steel sheets are used for exhaust manifolds, mufflers and other exhaust system members that require high-temperature strength and oxidation resistance. As these members are manufactured by press-forming, the steel sheets should have press formability.
Meanwhile, the service temperature for these members rises year after year. To cope with this temperature rise, it has been preferable to enhance the high-temperature strength of the material steel sheets by increasing the addition of Cr, Mo, Nb and other alloying elements.
The addition of alloying elements by simple manufacturing methods, however, has at times, lowered the workability of material steel sheets to such a level as to make press forming likely impossible.
Increasing the cold reduction ratio is conducive to effectively increasing the “r” value that is an index of press formability of steel sheets. However, the material steel sheets for such exhaust system members are relatively thick (e.g., between approximately 1.5 mm and 2 mm). Therefore, the conventional manufacturing processes that limit the thickness of cold-rolled strip to within a certain range do not permit securing sufficient cold reduction ratios.
In order to solve the above-described problem by increasing the “r” value, which is an index of press formability, without impairing the high-temperature properties, various studies have been made regarding the chemical composition and manufacturing method of steel sheets.
Conventionally, the workability of Cr-bearing heat-resistant steels has been improved by adjusting the chemical composition as described in, for example, Japanese Patent Publication No. 09-279312. However, composition adjustment alone may not be enough to solve the problems, such as cracks caused by pressing, in thicker materials manufactured with relatively low reduction ratios.
Japanese Patent Publication No. 2002-30346 describes a method that specifies the optimum hot-rolled strip annealing temperature based on the relationship between the hot-rolling starting and finishing temperatures, Nb content and annealing temperature. However, the specification of the hot-rolled strip annealing temperature alone is sometimes not enough where there are effects of elements (C, N, Cr, Mo, etc.) that are related to Nb-bearing precipitates.
Japanese Patent Publication No. 08-199235 describes a method that applies aging treatment to hot-rolled steel strip for more than one hour. This method, however, has a drawback that commercial manufacturing efficiency is extremely low.